Brazing is a common technique used to join two close-fitting surfaces. It involves placing a filler material such as a metal or metal alloy in the junction between two surfaces and applying heat so that the filler material melts. The filler material can then flow at least partially into the junction and solidify to form a brazed joint, thus bonding the two surfaces tightly together. The filler material and the material of the surfaces often partially dissolve one another at the interface, leading to strong bonding at a molecular level.
In laser brazing, the filler material is heated with a laser, which can provide accurate, controllable heating. However, one problem with current laser brazing processes is that, upon cooling and solidifying, the filler material forms pores, some of which can break the surface of the brazed joint. With conventional laser brazing processes, it is estimated that between 2 and 4% of the brazed joint comprises surface-breaking pores. Porosity is thought to be caused by small impurities in the filler material, impurities or defects in the surfaces to be joined and the laser brazing process itself. By lowering the impurities in the filler material and the contamination of the surfaces, the percentage of surface-breaking pores can be lowered to around 2%. Surface-breaking pores can be eliminated by mechanically working the brazed joint (e.g. polishing, grinding or filling), but this technique is time-consuming and labour-intensive. Pores in the brazed joint will tend to have negative effects on the physical properties (e.g. strength) of the joint, and surface-breaking pores are a particular problem in applications requiring a smooth finish. Furthermore, in conventional laser brazing, rings or herring-bone patterns can form on the surface of the filler material as it cools.
JP 7211424 discloses a soldering technique in which a laser beam is split so that a workpiece having high heat capacity is pre-heated by a first laser beam before being soldered by a second laser beam. This avoids problems associated with sharp partial preheating of the workpiece.
JP 63168272 discloses a method for removing pinholes in brazed parts by using a laser heat source to remelt a defective fused zone.
US 2004/009964 discloses a system for laser beam welding comprising a laser beam welder and a TIG welder, wherein the TIG welder is used for re-melting a portion of the bead obtained from the laser welder.
However, a need remains for an efficient method for laser brazing in which pore-formation and undesired surface patterns in the brazed joint can be reduced, in particular in which the presence of surface-breaking pores can be reduced or eliminated entirely. This is especially relevant for brazed joints which are to be painted, as surface irregularities or surface-breaking pores may also be evident after painting. The method should provide a simple, economical solution and should be easily compatible with existing processes, particularly the high-speed processes used for mass-produced goods.